In recent years, a magnetic random access memory (MRAM) has been proposed and studies of MRAM have been actively made toward practical application. The magnetic random access memory operates as a nonvolatile random access memory by using a magnetic body as a storage element, ensures rewrite resistance of 1015 times or more and can perform switching on a time scale of a few nanoseconds or less. In consideration with these conditions, there is demand that the magnetic random access memory can act as a high-speed nonvolatile random access memory operating at a few 100 MHz or higher.
The magnetic random access memory is formed of a magnetoresistive effect element. The magnetoresistive effect element includes a magnetization free layer, an insulating layer and a magnetization fixed layer, and the magnetization fixed layer is generally adjacent to an antiferromagnetic layer. The magnetization free layer, the insulating layer and the magnetization fixed layer are laminated in this order to form magnetic tunnel junction (MTJ). The magnetization fixed layer includes a relatively hard ferromagnet and its magnetization direction is fixed at substantially one direction by the antiferromagnetic layer provided adjacent to the magnetization fixed layer. The magnetization fixed layer acts as reference in reading. Meanwhile, the magnetization free layer includes a relatively soft ferromagnet and magnetic anisotropy is provided so that its magnetization direction may be parallel or antiparallel to magnetization of the magnetization fixed layer. The magnetization free layer functions as an information storage site. The insulating layer is made of an insulating material. Information “0” or “1” is stored in the magnetic random access memory depending on whether magnetization of the magnetization free layer is parallel or antiparallel to that of the magnetization fixed layer.
To read the information in the magnetic random access memory, a magnetoresistive effect is utilized. That is, the information is read by passing a current passing through the MTJ and detecting a difference between resistance values of MTJ due to a difference between magnetization of the magnetization free layer and magnetization of the magnetization fixed layer in relative angle.
Meanwhile, various methods of writing information into the magnetic random access memory have been proposed. The methods are broadly divided into a magnetic field write method and a spin injection write method.
According to the spin injection write method, by changing a direction of the current passing through the MTJ, the magnetization of the magnetization free layer is reversed through spin torque transfer with respect to the magnetization of the magnetization fixed layer. According to the spin injection write method, a current required for writing is proportional to the area of the MTJ. Accordingly, as the area of the MTJ is smaller, the value of the current required for writing becomes smaller. In other words, the spin injection write method has a good scaling property and is expected as means capable of achieving a large-capacity magnetic random access memory.
However, according to the spin injection write method, a relatively large current passes through the MTJ in writing, possibly resulting in poor rewrite resistance property. Withstanding pressure of the insulating layer is also cited as an applicational problem. Furthermore, spin injection magnetization reversal takes relatively longer time than magnetization reversal by magnetic field, which is disadvantageous for a high-speed operation. In other words, there are various problems in realizing a high-speed and high-reliability random access memory according to the spin injection write method.
On the other hand, magnetization reversal by a magnetic field occurs in a nanosecond or less, and since no large current passes through the insulating layer, reliability is assured, reliability is assured. Thus, for the magnetic random access memory capable of operating at high speed, it is desired to use the magnetic field write method.
Generally, according to the magnetic field write method in the magnetic random access memory, a magnetic field induced when a current passes to a write interconnection disposed in a vicinity of the MTJ is utilized. In most magnetic random access memories according to currently studied and developed magnetic field write methods, a magnetoresistive effect element is disposed at an intersection of two write interconnections orthogonal to each other and writing is performed by a synthetic magnetic field applied to the magnetization free layer when a current passes to the two write interconnections (hereinafter referred to as a biaxial write method).
The most common biaxial write method is an asteroid method. According to this method, a current passes to two write interconnections which are orthogonal to each other simultaneously and the magnetization free layer is magnetically reversed by the synthetic magnetic field. At this time, the magnetic field induced by one write interconnection is applied to a cell located in a same column or row as a selected cell, causing a so-called semi-selected state. To prevent magnetization reversal in the semi-selected state, recording must be performed within a limited margin. In other words, the asteroid method has a problem in cell selectivity.
Document 1 (specification of U.S. Pat. No. 6,545,906) proposes a toggle method as a biaxial write method solving the cell selectivity problem. According to the toggle method, a current sequentially passes to the two write interconnections which are orthogonal to each other, thereby causing magnetization reversal. Although the toggle method can solve the cell selectivity problem substantially completely, reading must be performed before writing, which is unsuitable for high-speed operation.
Document 2 (Japanese Laid-Open Patent Application JP-P2004-348934A) proposes a uniaxial magnetic field write method. According to this method, the above-mentioned problems in selectivity and high-speed property can be solved simultaneously. According to the uniaxial magnetic field method, one cell has one write interconnection and the write interconnection is connected to the source/drain of the MOS transistor. A gate of the MOS transistor is connected to a word line provided along a first direction and the other source/drain of the MOS transistor is connected to a bit line provided along a second direction. With such configuration, the problems in cell selectivity and high-speed property can be solved simultaneously. Thus, it can be said that the uniaxial magnetic field method is a desirable method for achieving the magnetic random access memory capable of performing a high-speed operation.